The present invention is related to the field of electrical connections. In particular, one embodiment of the present invention provides an improved method and apparatus for making electrical contacts to an electro-optic modulator.
Optical modulators are well known to those of skill in the art and are commonly used to modulate light in a variety of applications including, for example, laser scanning for non-impact printing, transmission of data in optical fibers, screen displays, film exposure, optical reading equipment, optical fourier transform generators, and the like. Many optical modulators operate by passing a light beam, usually a monochromatic coherent beam of light, into or through a crystal. An electrical field is imposed on the crystal via, for example, a set of electrodes on a surface of the crystal. The electrical field varies optical properties of the crystal such its index of refraction. As the index of refraction of the crystal varies, the crystal modulates the light in a desired manner through variation of the electrical field.
One such optical modulator is described in greater detail in, Sprague et al., "Linear Total Internal Reflection Spatial Light Modulator For Laser Printing," SPIE Vol. 299 (1982) which is incorporated by reference herein for all purposes. Related modulators are described in U.S. Pat. Nos. 4,391,490, and 4,718,752, which are also incorporated by reference herein for all purposes.
While prior art crystals have met with significant success, substantial problems still remain. For example, the electrical field in the modulator disclosed in Sprague et al. is imposed by electrodes on a driver crystal which are mechanically engaged to the modulator crystal. This arrangement provides for poor contact to the modulator crystal and, therefore, poor response characteristics.
As an alternative to direct mechanical engagement of linear electrodes on the driver chip to the modulator crystal, linear electrodes could be formed on both the driver chip and the modulator crystal and subsequently pressed together. Long electrodes would provide a long interaction region for light and ease alignment problems. The use of matching linear electrodes on both the driver chip and modulator crystal also presents difficulties however. For example, since a large number (2-50,000 and commonly 500-10,000) of the control electrodes are required on the modulator crystal and since the electrodes are of necessity very small (on the order of 1 to 10 microns wide) and separated by a very small distance (also on the order of 1 to 10 microns) it is exceedingly difficult to properly align the integrated circuit driver chip electrodes to the crystal electrodes and often the electrodes of the driver chip would be crossed with the electrodes of the underlying crystal, resulting in unsatisfactory device performance.
Additionally, any asperites from contamination or localized non-uniformity in substrate processing when mechanically compressed form pressure-induced variations in optical field properties which conflict with the uniform operation of the modulation device. Further, the small size of the electrodes renders them easily damaged, destroyed, or electrically shorted to other electrical contacts in the process of forming the compression fitting. Still further, the linear arrangement of the electrodes results in either a very long, narrow driver chip (which becomes difficult to produce and, therefore, excessively expensive) or the need to apply multiple driver chips (which dramatically increases the magnitude of the alignment problems). Still further, matching linear electrodes could not practically be repaired due to the damage incurred in unseating and re-seating the small electrodes. Still further, the matching linear electrode arrangement would result in the need for a cantilevered driver chip design in order to provide an interface to the outside world. The cantilevered arrangement would often result in chip breakage, and the like. Of course, even when such linear electrodes are properly aligned, the coefficient of thermal expansion of the silicon driver chip and the crystal differ by a significant amount and when heated the electrical contacts may be displaced sufficiently to destroy electrical contacts, or short to neighboring electrodes, or smear the metal electrodes on either or both the driver chip and the crystal.
From the above it is seen that an improved electrical contact for an optical modulator and method of forming electrical contacts on an optical modulator are desired.